Resonant aero-acoustic excitation of cavity depth modes

Abstract

Results are generalized of parametric experimental investigations of resonant aero-acoustic excitation of the depth modes of a flow-grazed cavity at low-subsonic (U50 m/s) and transonic flows (0.7M1.045). Special attention is given to the fundamental mechanisms responsible for this phenomenon. The conditions for these mechanisms to manifest themselves and interact are predicted. Observations are made of the relations between the characteristic flow and cavity parameters at which the maximum resonant aero-acoustic response of the cavity depth mode takes place. These relations take into account the distinction between the flow velocity and the characteristic spreading velocity of the disturbances downstream of the wall discontinuity as well as the distinction between the cavity depth and the characteristic linear scale in the Helmholtz number connected with the edge effect for sound waves in the vicinity of the cavity gap. Experimental data illustrating the reliability of these relations are presented. The extent of the Strouhal-number region where the cavity mode excitation can be treated as an aero-acoustic resonance is determined.